1. Field of the Invention
This invention relates to the field of grinding or comminution and dispersion and more particularly to the reduction of solid matter into fine particles.
The reduction of solid matter into fine powders is a major task of an industrial society. As an example Portland cement is made from finely ground limestone, clay or shale, sand, and coal or other fuel. The limestone, clay or shale, and sand are subjected to a thermal process in which the heat is derived from the coal and the results are clinkers of material that must again be ground to produce the cement. Gypsum, after calcining, must be ground to produce sheet rock and other such products.
The food industry grinds many products including wheat, corn, rice, spices, sugar, and even chocolate. Paints, inks, and so forth use ground pigments and in turn undergo a dispersion process to disperse the ground pigment in a suitable vehicle.
Ceramics are made from finely ground materials. Generally the better the grind the better the ceramic product. Metals are ground as part of powder metallurgy and to prepare metallic pigments.
2. Description of the Prior Art
One of the oldest and simplest methods of grinding materials to fine powders uses a ball mill that generally consists of a horizontal cylindrical chamber that may be of any size. Ball mills have been constructed in sizes of up to eighteen feet in diameter by fifty or sixty feet long.
For many applications the ball mill is about half full of steel or ceramic balls in addition to the material to be ground. The balls roll over one another and aid in the grinding process.
In cases where the material to be ground, such as a paint, consists of a fine pigment to be dispersed, the balls, usually called the grinding media, are essential to the process while in other cases such as the grinding of cement clinker, the media is omitted. In this latter case the larger clinkers act as media for the smaller ones and a means is usually provided for extracting only the finer particles from the mill.
While the ball mill is effective and reliable, it tends to be large and slow. The physical size of the mill tends to cause it to be high in capital cost for the amount of work done.
There is much art having to do with overcoming the deficiencies of the ball mill. Alternative approaches to the task of grinding include mills wherein a material is stirred with media by means of mandrels. In another approach the material being ground in a liquid carrier is subjected to high shear rates by high speed blades or by being forced through narrow gaps between rapidly moving surfaces. These devices are most useful for dispersion while a ball mill both grinds and disperses.
In yet other attempts to obtain the benefits of a ball mill while overcoming its deficiencies, considerable prior art has addressed planetary mills in which the grinding chamber is orbited about an axis parallel to the axis of the grinding chamber. In such art the planetary motion imparts a centrifugal force that aids the action in the grinding chamber.
In U.S. Pat. No. 5,029,760 I describe a system wherein a rotatable drum assembly carries two rotatably mounted grinding tubes that are constrained to have no net rotation with respect to the base of the machine. Access to the grinding tubes may be made only to their ends and only one of the two grinding tubes may be addressed from either end. A second embodiment uses a series of four rotating wheels that drive two oppositely mounted frames that each carry one or more grinding tubes.
In U.S. Pat. No. 5,205,499 I describe a rotatable drum assembly that carries a single grinding tube that has advantages of permitting access to the grinding tube from both ends.
In a separate application of even date herewith entitled, "Improved Planetary Grinding Apparatus", I have described an improvement to the systems described in both of these previous patents wherein the improvement permits more ready access to the grinding tubes so that continuous feeding of the device is easier and more flexible than with previous methods.
Ball mills and planetary mills have in common that both use cylindrical grinding chambers that rotate in a force field that is perpendicular or almost perpendicular to the axis of rotation. This force field, in the case of a ball mill, is the gravitational field of the earth and, in the case of a planetary mill, is the centrifugal field generated by the planetary motion. Because of the equivalence of forces due to acceleration and gravitation, it can reasonably be said that ball mills and planetary mills both operate due to inertial fields.
It is advantageous to operate both ball and planetary mills in a continuous manner, with continuous feed being especially important for planetary devices. With ball mills the volume of the mill tends to be quite large, and the time required for grinding long. In a batch mode, that is, where the device is filled, run and then emptied, then refilled and so forth, ball mills might be loaded once a day and allowed to grind for twenty four hours. Though the time required for grinding is long, since the mill is large, significant production takes place and loading and unloading the mill is not an undue burden.
Still, even in the case of ball mills, it may be convenient to operate the mill continuously and to feed and withdraw material as steady streams. In such a case the mill needs almost no attention and production takes place with minimal labor content.
With a planetary mill, however, the volume of the mill tends to be small and the time for grinding only a few moments. If continuous feed is not used, much of the operating cycle may be spent in starting and stopping and loading and unloading the mill. In the case of a planetary mill it is usually a practical necessity that the device be continuously fed.
As has been discussed at length in my previous patents, listed above, it is possible to construct planetary mills that achieve continuous feed and withdrawal of the material being ground without rotating seals. Ball mills, on the other hand, appear to require such seals. Fortunately ball mills rotate at a relatively slow rate so that rotating seals are not much of a problem with continuous feed systems for such mills.
Two major problems exist with continuous feed systems. The first is that since the mill is usually a single cylinder, material that is insufficiently ground may by-pass grinding in the mill and be found in the output so as to compromise the grind. This problem can be overcome by making the mill long compared to its diameter, or by dividing the mill into separate connecting chambers so that equilibration tends to take place in stages (such an arrangement is sometimes called a tube mill).
The second problem has to do with the separation of media used for grinding in the mill from the material being ground. Especially in the case of planetary mills, wherein the grinding tube may be quite small in volume compared to the volume of the material going through the mill per unit time, there is a tendency for media to be carried along with the ground material. Since any media retained with the ground material tends to be a major problem, precautions need be taken to prevent media from being mixed with the product from the mill.
This problem has become especially severe in recent times because demand for ever finer grinds has led to the use of ever finer media in milling machines in general. This invention addresses this particular problem by providing for helical chambers at each end of a mill that tend to screw media back toward the center of the mill while permitting free flow of the material being ground.